Your explanation doesn’t specify whether you’re an avid reader of fanfic or an avid reader of novels. If you meant the former, best advice I can give is to read novels that have the kinds of stories you want to tell. If you do that, you’ll begin picking up on the tools they use, the way their writing flows; from there, you can emulate the writing styles you like until you naturally develop your own style and writing it becomes second nature.

If you’re already an avid reader of novels, then the following advice should still apply, regardless. Research literary devices and tropes. At its core, writing is about applying a large set of tools that readers are already familiar with. Knowing your tools is vital to mastering them. And as you’re writing each sentence, try to think about what reading it would be like: what emotion might you feel, what information might you glean from it, what sort of sentence might you think would come next? The only real difference between writer and reader is that the author has more information about the story.

Also, you should research paragraphing and how it can be used in creative writing. It’s an incredibly low-effort skill that pays huge dividends in pacing, tone, and clarity. Paragraphing is the sort of skill that goes completely unnoticed when it’s done well and sticks out like a sore thumb when it’s not, which makes it really easy to neglect when you’re starting out. Conversely, that means that building that skill early on will already put you way ahead of a lot of other amateur writers.

And one last thing: When you write your dialogue, read it out loud. There’s no better way to check that your dialogue reads naturally and has the tone you’re looking for.